1. Field of the Invention
This invention generally relates to a source driver and a source driving method, and more particularly to a source driver and source driving method for LCDs.
2. Description of the Related Art
FIG. 1 is a conventional driving circuit for an active matrix LCD (liquid crystal display) device 100. The LCD device 100 includes an LCD panel 110 having a TFT (thin film transistor) array 112 disposed thereon, a gate driving circuit 120 and a source driving circuit 130. The TFT array 112 is formed by a plurality of thin film transistors 113. Each transistor 113 has its gate 113a connected to a corresponding scanning line 114, its source 113b connected to a corresponding data line 116, and its drain 113c connected to one terminal of a corresponding display capacitor 118. The other terminal of the display capacitor 118 is connected to a common voltage VCOM. The gate driving circuit 120 is used for providing switching signals (i.e. scanning signals) to the scanning lines 114, and the source driving circuit 130 is used for providing level voltages to the data lines 116.
FIG. 2 is a schematic diagram of a typical source driving circuit 130 for the active matrix LCD device 100. The source driving circuit 130 comprises a voltage divider 200, a plurality of decoders 202 and a plurality of drivers 204. The voltage divider 200 is composed of resistors R1 to Rn and used for generating multiple level voltages. The level voltages generated from the voltage divider 200 are selected by switching the switches 202a in the decoder 202 and outputted to the inputs 204a of the drivers 204. Each driver 204 is respectively corresponding to each data line 116 of the LCD panel 110 (shown in FIG. 1), and connected to and drives each data line 116 through the output 204b. 
FIG. 3 is a schematic circuit of a driver 204 disclosed in U.S. Pat. No. 6,567,327 B2. The driver 204 comprises a pull-high differential amplifier 210, a pull-low differential amplifier 212. The driver 204 has an input 204a for receiving a level voltage Vin and an output 204b. The output voltage Vout of the driver 204 is fed back (negative feedback) to the inputs Vin− (i.e. inverting inputs) of the differential amplifiers 210, 212, and the level voltage Vin is inputted to the inputs Vin+ (non-inverting inputs) of the same.
The pull-high differential amplifier 210 is operated just while the output voltage Vout is smaller than the voltage at the input Vin+, whereby increasing the output voltage Vout toward the voltage at the input Vin+. In addition, the pull-low differential amplifier 212 is operated just while the output voltage Vout is larger than the voltage at the input Vin+, whereby decreasing the output voltage Vout toward the voltage at the input Vin+.
The operation of the driver 204 is described below. The output voltage Vout is stable while the voltage at the input Vin+ equal to that at the input Vin−. When the voltage at the input Vin+ is changed and larger than that at the input Vin−, that is, when the level voltage Vin is larger than the output voltage Vout, only switches S1, S2, S3 are turned on such that the transistor 220 is turned on by an output voltage V01; then, the output voltage Vout begins increasing toward the voltage at the input Vin+; finally, only switch S0 is turned on such that the input 204a is short to the output 204b whereby more precisely pulling the voltage level of the output voltage Vout to that of the level voltage Vin. In addition, When the voltage at the input Vin+is changed and smaller than that at the input Vin−, that is, when the level voltage Vin is smaller than the output voltage Vout, only switches S4, S5, S6 are turned on such that the transistor 222 is turned on by an output voltage V02; then, the output voltage Vout begins decreasing toward the voltage at the input Vin+; finally, only switch S0 is turned on such that the input 204a is short to the output 204b whereby more precisely pulling the voltage level of the output voltage Vout to that of the level voltage Vin.
However, when the voltage level of the output voltage Vout is close to the voltage level of a high supply voltage VDD and smaller than that of the level voltage Vin, it is difficult for the pull-high differential amplifier 210 to pull up the output voltage Vout. In addition, when the voltage level of the output voltage Vout is close to the voltage level of a low supply voltage VSS and larger than that of the level voltage Vin, it is difficult for the pull-low differential amplifier 212 to pull down the output voltage Vout. Therefore, the output voltage Vout of the driver 204 is limited and cannot cover the whole voltage range between VSS and VDD.
Accordingly, the present invention provides a source driver for LCDs having a wide driving voltage range so as to solve the above-mentioned problem existing in the art.